The procedure for the manufacture of hot rolled steel sheet by means of a continuous hot rolling mill process has a much higher productivity than the procedure for the manufacture of thick steel plate. The steel sheet product obtained by the former procedure is also very excellent in the shape, dimensional accuracy and surface condition as compared with steel plate obtained by the latter procedure. Thus steel sheet manufactured by a continuous hot rolling mill amounts to as much as about 40% of the total hot rolled steel material and has wide applications in various field. For example, hot rolling has recently been used not only for 30 kg/mm.sup.2 class steel but also for steel having high strength and good toughness. However, the finishing or coiling temperature of the hot rolling procedure which has a great effect upon the properties of hot rolled steel sheet cannot be significantly modified in view of manufacturing facilities with the result that the existing of the material as rolled and subject to undue properties unavoidable limitation. Accordingly, it has heretofore been a practice for accomplishing the above object to use an expensive alloy or, if necessary, conduct a controlled rolling. The steel sheet thus obtained, however, becomes very expensive and moreover gives rise to a considerable decrease in productivity due to the controlled rolling. The as-hot-rolled steel sheet also has the disadvantage of fluctuating in quality along the longitudinal length of the coil.
In addition to the production of as-hot-rolled steel sheet having high strength and good toughness, it has conventionally been practiced in the manufacturing procedure for a steel plate to effect such heat treatments such as quenching, tempering, normalizing, etc. After hot-rolling. In this procedure, however, the heat treatments are conducted with respect to a sheet material which has been cut to length, which thus results in a defect as to shape. Thus it has been reported that a product having a thickness up to 6mm can hardly be manufactured. Even if such a product can be manufactured, it is necessary to effect strong cold-straightening after the heat treatment, which results in a considerable deterioration in the material. Such problem also remains even in steel having a thickness of 6 to 10mm.
CONVENTIONALLY, COLD ROLLED STEEL SHEET HAS BEEN SUPPLIED AS A STEEL SHEET SATISFYING THE REQUIREMENTS FOR DEEP DRAWING. In an attempt to contract the annealing and skin-pass rolling process for cold rolled steel sheet, it has recently been proposed to effect a continuous annealing process. However, it is often difficult in continuous annealing to obtain a desirable texture for deepdrawability due to the rapid heating, especially in the cases of Al-killed steel and rimmed or capped steel. Such cold rolled steels, when intended for deep drawing, must be subjected to further steel-making operations such as vacuum degassing, and the like.
As for hot rolled steel sheet, it has been a desideratum to develop such a sheet having drawability equivalent to that of cold rolled steel sheet or a hot rolled steel sheet having high strength with drawability. Also a demand for a very thin hot rolled steel sheet having a thickness not more than 1.6 mm is increasing year after year. In a conventional hot rolling mill, it is difficult to produce steel sheet having a thickness not more than 1.6 mm. The main reasons for this are that it is difficult to maintain the finishing temperature above the A.sub.3 point for a drawing steel and to maintain a good shape for a high strength steel.
It is therefore an object of the invention to provide a method which is conducted upon a continuous heat treatment of hot rolled strip coil and wherein a stress is positively imparted.
It is another object of the invention to make it possible to conduct the heat treatment upon a thin material having a thickness up to 6 mm which has heretofore been impossible.
It is still another object of the invention to make it possible to manufacture hot rolled steel sheet or strip with a thickness not more than 1.6 mm having a good shape, high strength and good toughness.
It is a further object of the invention to provide a product of uniform quality along the entire length of the coil.
It is still a further object of the invention to provide a product of low cost as compared with the as-rolled high tensile strength steel in which a highly expensive alloy element must be used.
It is still a further object of the invention to make it possible to omit or lessen the controlled rolling of the material during the hot rolling.
It is still a further object of the invention to obtain a high strength and good toughness steel, which has never been manufactured, by means of controlled cooling after heating.
It is still a further object of the invention to make possible the manufacture of a drawing quality of cold rolled Al-killed, rimmed or capped steel sheet by continuous annealing by means of a preliminary heat treatment of hot coil.
It is still a further object of the invention to make possible the manufacture of quality of drawing hot rolled steel sheet which has not been obtained in the prior art.